We and others have established that the endothelium derived nitric oxide-guanylyl cyclase-cGMP pathway is an important mechanism of vasodilation in the pulmonary circulation during the transition from fetal to newborn state. Our laboratory was first to establish that cGMP-dependent protein kinase (PKG) is critically important for mediating the action of cGMP in the pulmonary circulation. We have also shown that cAMP can cross-activate PKG and thus it appears that PKG mediates the vasodilator action of both cyclic nucleotides. An important signal for the pulmonary vasculature to dilate at birth is the abrupt increase in oxygen tension. We have data to show that when fetal pulmonary vessels are exposed to increased oxygen, there is an increase in PKG activity leading to an enhanced dilator response to cGMP. Our preliminary data show that exposure of ovine fetal pulmonary vascular smooth muscle cells (PVSMC) to increased oxygen results in: 1. An increase in PKG kinase activity, 2. An increase in PKG protein levels, as well as 3. An increase in PKG mRNA levels. Our hypothesis is that at birth, oxygen up-regulates PKG protein levels and PKG kinase activity, thereby facilitating pulmonary vasodilation and a smooth neonatal transition. Our aims are: 1. To investigate how oxygen regulates PKG kinase activity in PVSMC; 2. To determine the effect of oxygen on PKG protein levels in PVSMC; and 3. To investigate two mechanisms of regulation of PKG protein and mRNA levels in PVSMC. Experiments will be done in ovine fetal PVSMC from arteries and veins. We will also use isolated ovine fetal intrapulmonary arteries and veins, to model the experiments with PVSMC, so that we can determine the physiological significance of the in vitro biochemical data. Our overall goal is to gain a better understanding of the unique biological processes by which pulmonary vasomotor tone is regulated in the perinatal period and to understand the role of PKG in this process. Knowledge derived from our studies should enable us to develop new prevention and treatment strategies for disorders of pulmonary vasomotor control at birth such as Persistent Pulmonary Hypertension of the Newborn.